compositing chroma video on plain in 3d space
Your question has a pretty broad and step intensive answer, but I'm writing this to help point you in the right direction, instead of even trying to give you some sort of magic wand to accomplish what you are asking.

I believe that your answer is to do the keying first, and then get a solid color as your background. You might have to spend a lot of time here as this is an art, and may even require rotoscoping/masking at times, and edge cleanup of your focal subject. If at all possible, you will do most of this work in your footage lighting to reduce the amount of keying problems you would have after the footage is taken.

Once you have a clean video, then your cycles engine can help you accurately isolate your intended subject in 3D space (In pseudo - Real Time).

You have to put in the work somewhere.
Cycles is pretty powerful, and can go pretty far, however there is far more power in cleaning up your video first. Then you can use this solid background as a decision factor to mix between video color, and a transparency shader node with a high degree of reliance.

Luckily Blender has the tools to do the cleanup both in the Video Sequence editor for masking, and in the compositor to apply the "VSE" mask and do your chroma/color matte keying.

Hopefully this should point you in the right direction.

Here's a link that I though would help as well:
Youtube video showing how both green-screen and masking works

Here's a blend that I put together real quick to show you what I mean by making decisions with color in the cycles engine. The two "colored" images are packed in the file for your study.